Factors determining the spread and impact of the exotic grass Indian couch (Bothriochloa pertusa) into native ecosystems

Abstract

The invasion by exotic plant species into native plant communities is a major threat to biodiversity and ecosystem function across the globe. In many cases, their widespread establishment and spread is facilitated by their deliberate introduction for pasture, lawn and soil conservation. This thesis discusses the impact, spread and management of invasive pasture species within Queensland, Australia, with a particular focus on the exotic grass species Bothriochloa pertusa (Indian couch). Bothriochloa pertusa was introduced in 1939 and in recent decades its spread and observed dominance throughout the landscape has raised concerns about the impacts of this species on native biodiversity. I investigated the impacts of B. pertusa on floristic diversity and discuss these results in the context of Queensland’s recent and rapid intensification of land-use.In Chapter 2, I used comprehensive mapping data and results from field surveys to investigate changes in land-use and the cover of prominent invasive species over time (1996 – 2018), within a focus area of Queensland. I found that despite policy developments aimed at reducing land-clearing and protecting threatened ecosystems during this time, land-clearing still occurred at a rate of 36,960 ha per year in the study area, mainly for the expansion of grazing and cropping agricultural systems. Remaining fragments of native vegetation experienced a significant increase in the cover and occupancy of the invasive pasture species, B. pertusa and Cenchrus ciliaris (buffel grass). I discuss the role of propagule pressure altered disturbance regimes and habitat loss in promoting the spread of these species in Queensland. The results from this chapter highlight the significant and ongoing impacts of land-use change for native biodiversity. Further they highlight the need to implement consistent, effective and well-regulated policies to protect species and habitats in the future. In Chapter 3, I present a comprehensive account of the introduction history of B. pertusa and discuss factors associated with its spread throughout the landscape. Using B. pertusa presence/absence data I also built habitat suitability models, to predict its potential distribution and cover within Queensland. Results from this chapter highlight climatic extremes and livestock grazing as key mechanisms facilitating the establishment and spread of B. pertusa. In Chapter 4, I used fence-line comparisons, within two conservation reserves and their neighbouring pastoral properties, to further assess the role of grazing in facilitating the spread of B. pertusa. Within both reserves and across the three vegetation types examined, the cover of B. pertusa was higher in the grazed pastoral property than the conservation reserve. Diversity did not differ significantly between the pastoral properties and reserves, although this result was likely confounded by high C. ciliaris cover within the un-grazed reserve. Results suggest grazing protected areas and conservative stocking rates can reduce the spread and impact of B. pertusa within Queensland and elsewhere in Australia. Other invasive species however, such as C. ciliaris may be favoured by grazing protection. In Chapter 5, I comment on the use of controlled livestock grazing to manage the invasive grass Cenchrus ciliaris and discuss the implications of this for the spread of B. pertusa.In Chapter 6, I investigated the impact of B. pertusa on native floristic diversity in the iron-bark woodlands of sub-coastal Queensland, a habitat particularly vulnerable to invasion. Specifically, I assessed its impact across multiple spatial-scales and found that diversity was lower at both small and large spatial scales in invaded areas. By examining changes to root-traits with increasing invader cover we found that competition for belowground space and resources was a likely mechanism contributing to its broad-scale impact on diversity. Results from these two chapters highlight the capacity for B. pertusa to continue to spread and intensify in Queensland, with the potential for significant and large-scale declines in floristic diversity.In Chapter 7, I bring together my results and those of previous studies, to provide recommendations and discuss strategies to reduce the impacts of B. pertusa, but more broadly the impacts of invasive pasture species and land-use change. With the majority of land-used for livestock grazing, ensuring the implementation of adaptive and conservative grazing strategies is crucial for maintaining ecosystem resilience and improving resistance to invasion and stochastic events. Further, encouraging the protection and restoration of habitat by ensuring its economic viability for producers is essential and there is considerable scope for this to be achieved (i.e. premium carbon credits, environmental stewardship). For the successful application of these initiatives however, they must be coupled with ongoing research and adequate financial, technical and administrative support.This thesis examined the spread and impact of invasive pasture species and in doing so highlighted the interactions between land-use change, the spread of invasive species and biodiversity decline. The findings presented here suggest the current approach to land-use is not compatible with the conservation of native species and habitats, and considerable changes are necessary if we are to ensure their persistence in the future.